debian-mirror-gitlab/doc/development/database/setting_multiple_values.md

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Setting Multiple Values

Introduced in GitLab 13.5.

There's often a need to update multiple objects with new values for one or more columns. One method of doing this is using Relation#update_all:

user.issues.open.update_all(due_date: 7.days.from_now) # (1)
user.issues.update_all('relative_position = relative_position + 1') # (2)

But what do you do if you cannot express the update as either a static value (1) or as a calculation (2)?

Thankfully we can use UPDATE FROM to express the need to update multiple rows with distinct values in a single query. One can either use a temporary table, or a Common Table Expression (CTE), and then use that as the source of the updates:

with updates(obj_id, new_title, new_weight) as (
  values (1 :: integer, 'Very difficult issue' :: text, 8 :: integer),
         (2, 'Very easy issue', 1)
)
update issues
  set title = new_title, weight = new_weight
  from updates
  where id = obj_id

The bad news: there is no way to express this in ActiveRecord or even dropping down to ARel. The UpdateManager does not support update from, so this is not expressible.

The good news: we supply an abstraction to help you generate these kinds of updates, called Gitlab::Database::BulkUpdate. This constructs queries such as the above, and uses binding parameters to avoid SQL injection.

Usage

To use this, we need:

• the list of columns to update
• a mapping from object/ID to the new values to set for that object
• a way to determine the table for each object

For example, we can express the query above as:

issue_a = Issue.find(..)
issue_b = Issue.find(..)

# Issues a single query:
::Gitlab::Database::BulkUpdate.execute(%i[title weight], {
  issue_a => { title: 'Very difficult issue', weight: 8 },
  issue_b => { title: 'Very easy issue', weight: 1 }
})

Here the table can be determined automatically, from calling object.class.table_name, so we don't need to provide anything.

We can even pass heterogeneous sets of objects, if the updates all make sense for them:

issue_a = Issue.find(..)
issue_b = Issue.find(..)
merge_request = MergeRequest.find(..)

# Issues two queries
::Gitlab::Database::BulkUpdate.execute(%i[title], {
  issue_a => { title: 'A' },
  issue_b => { title: 'B' },
  merge_request => { title: 'B' }
})

If your objects do not return the correct model class (perhaps because they are part of a union), then we need to specify this explicitly in a block:

bazzes = params
objects = Foo.from_union([
    Foo.select("id, 'foo' as object_type").where(quux: true),
    Bar.select("id, 'bar' as object_type").where(wibble: true)
    ])
# At this point, all the objects are instances of Foo, even the ones from the
# Bar table
mapping = objects.to_h { |obj| [obj, bazzes[obj.id]] }

# Issues at most 2 queries
::Gitlab::Database::BulkUpdate.execute(%i[baz], mapping) do |obj|
  obj.object_type.constantize
end

Caveats

Note that this is a very low level tool, and operates on the raw column values. Enumerations and state fields must be translated into their underlying representations, for example, and nested associations are not supported. No validations or hooks are called.